In connection with the so-called “mass customization”, i.e. the production and sale of products individually adapted to the body measurements of a customer, the detection of the three-dimensional spatial shape of the body or of body parts is an important problem to be solved in engineering. When the three-dimensional spatial shape of a body part e.g. the torso/leg region of a customer, is known, individual articles of clothing such as pants, shirts, etc. may be manufactured with a very good fit.
A number of body scanners have been developed which detect and digitize the three-dimensional shape of the human body or of body parts contactlessly, using various optical methods, e.g. laser triangulation, stereo methods, moirémethods, etc., in which the data records describing the digitization are then made available for the automated production of individually adapted products. As a rule, such body scanners operate with technically very sophisticated and expensive processes which, in addition, require optical calibration and can therefore be employed only by trained specialist staff.
The so-called passive methods of short-range photogrammetry are considerably more cost-effective since merely calibrated cameras or digital cameras are required, rather than expensive projection units or precisely calibrated mechanical structures.
For instance, the Patent EP 0 760 622, “Sensing Process and Arrangement for the Three-Dimensional Shape in Space of Bodies or Body Parts”, the applicant and inventor of which is Robert Massen, discloses a method which allows an optical detection of the 3D coordinates of a body or body part, involving very little technical expenditure. To this end, the body is covered with an elastic envelope specially marked with precisely located point markers, and photographs of the body are taken from a plurality of camera positions that overlap but are otherwise taken free-handed. By a photogrammetric evaluation of the corresponding point markers in the individual overlapping image areas of the photographs, a 3D data record of the body part may be established.
In order to allow a 3D reconstruction of the body part from the point markers provided on the body part or on an envelope pulled over the body part based on two overlapping image recordings using methods of photogrammetry, the pixel coordinates of the so-called homologous point markers, i.e. of the point markers mutually corresponding to each other in the images need to be determined. This process is also referred to as registration. Therefore, in an automatic registration, those point markers which correspond to each other need to be found from two overlapping image recordings using methods of two-dimensional image processing. In previous known methods, this is achieved by the use of individually encoded point markers. The point markers used are circles, for example, that are surrounded by radial segments representing a binary code (in this connection see the left-hand part of FIG. 1). Here, the center of the encoded marker defines the location of the photogrammetric point marker. The radial segments provide a unique characterization for each point marker by a code of its own.
In the prior art methods, attempts are made with the aid of automatic image processing to find such point markers in two images, to read their code and to prepare a list of the corresponding point markers, i.e. the homologous point markers. Once preparation of this list is complete, the known methods of image orientation and bundle adjustment may be employed to establish a 3D data record containing the XYZ space coordinates of all homologous point markers.
In the photogrammetric measurement of large structures such as vessel hulls, aircraft parts etc., the space required by such encoded point markers is not relevant. However, a disadvantage of the prior art methods for the photogrammetric determination of the 3D shape of an object, which make use of individually encoded point markers that cover a relatively large area consists in that these methods are not suitable for the digitization of very small objects, which need to be covered with many point markers due to the required spatial point density. When using encoded point markers as are illustrated in FIG. 1, for example, in such cases the space offered by the surface of the object is not sufficient for achieving the spatial point density necessary for a good photogrammetric measurement.
In German Patent Application Serial Number 100 25 922.7 entitled “Automatische photogrammetrische Digitalisierung von Körpern und Objekten” (“Automatic Photogrammetric Digitization of Bodies and Objects”), the applicant and inventor of which is Robert Massen, it is in addition proposed to provide area markers on the envelope described in EP 0 760 622, which each comprise a plurality of point markers and form a background of the point markers, the surface area of the area markers having a particular color. The use of color image processing methods allows an easy automatic determination of the area markers (regions) corresponding in the different photogrammetric images and then of the corresponding point markers (so-called homologous pixels) therefrom. Once the list of the homologous pixels is available, the 3D data of the entire body part may be calculated using the methods of image orientation and bundle adjustment that are familiar to a person of ordinary skill in the art of photogrammetry.
A drawback of this method is that a relatively large number of different colors are needed for marking. Therefore, the color fidelity of the cameras employed must be sufficiently high and stable to be able to adequately differentiate between such multitude of different colors in an automatic recognition. Also, the standards applied to constancy and color fidelity of the illumination are higher than those in a marking technique, which would manage with only very few colors that are distinctly different in the color space. A further disadvantage of this method resides in the relatively high geometric resolution required for optically resolving the color edges and color corners used as markers.
Both requirements, high color fidelity and high geometric resolution, can not be satisfied by simple and very low-priced cameras such as for instance by webcams based on CMOS image sensors or image sensors incorporated in future mobile telephones and so-called personal organizers (pocket computers). Thus, it is not possible for such very simple and inexpensive image recording devices to be used for a simple digitization of body parts as described above.
Therefore, a technical and economic interest exists in providing a method and an arrangement for the photogrammetric detection of the 3D shape of an object, featuring an improved marking technique that allows a low-cost and simple measurement even of small objects.
Furthermore, there exists an interest in a method and an arrangement for the photogrammetric detection of the 3D shape of an object that—with regard to the marking—manage with only few different colors and only relatively rough structures, in order to enable an automatic determination of homologous pixels from a plurality of overlapping image recordings using inexpensive, low color fidelity and low resolution cameras and imaging devices and with an illumination that may be of low definition in respect of the color. This would permit to employ the method even when using low-priced webcams (web cameras) or digital cameras, for example, that are commonly used today and are rather poorly specified as regards color fidelity and resolution.